The signal path between two telephones, involving a call other than a local one, requires amplification using a four-wire circuit. The cost and cabling required discourage extending a four-wire circuit to a subscriber's premise (i.e. Private Branch Exchange (PBX)) from the local exchange or Central Office (CO). For this reason, the four-wire trunk circuits are coupled to two-wire local circuits, using a device called a hybrid. Thus, when a PBX is connected to the CO through a Loop-Start (LS) trunk Line, the hybrid couples the analog signal from the four-wire circuit (where incoming and outgoing signals are separated) to the two-wire circuit where the incoming and outgoing signals are combined.
Unfortunately, by its nature the hybrid is a leaky device. As signals pass from the four-wire to the two-wire portion of the network, the energy in the four-wire section is reflected back, creating an echo of the signal. The intensity of the echo depends on how well the impedance is matched between both sides of the hybrid. The impedance of the two-wire circuit can vary wildly depending on factors including the line set-up in the CO equipment, the distance between CO and PBX, the electrical characteristics of the wire, etc. Provided that the total round-trip delay occurs within just a few milliseconds, the echo generates a sense that the call is ‘live’ by adding sidetone, thereby making a positive contribution to the quality of the call.
In cases where the total network delay exceeds 36 ms, however, the positive benefits disappear, and intrusive echo results. The actual amount of signal that is reflected back depends on how well the balance circuit of the hybrid matches the two-wire line. In the vast majority of cases, the match is poor, resulting in a considerable level of signal being reflected back.
It is known in the art to employ Line Echo Cancelers (LECs) to address hybrid echo cancellation in Voice-Over-IP (VoIP) systems. Most LECs use the well-known Normalized Least Mean Square (NLMS) algorithm to adapt a linear Finite Impulse Response (FIR) filter, so that the FIR filter matches the transfer-function of the echo path and provides a counter-signal to cancel the echo.
Because of the linear nature of the FIR filter and NLMS algorithm, LECs work well only if the echo path is truly linear. In reality, the LS trunk line circuit may contain some degree of nonlinear effects resulting from operating characteristics of power amplifiers and D/A, A/D converters, especially when a large signal (e.g. a loud speech signal) is present. Additional nonlinear sources include noise on the line, overshoot of line filters and quantization error of CODECs in digital systems. All of these sources create nonlinear components to the echo channel, which cannot be modeled by a linear FIR filter.
Moreover, adapting on such a nonlinear echo can result in a loss of divergence in a well-converged FIR filter, giving rise to annoying echo bursts before the FIR filter re-converges. The portion of the echo that cannot be canceled by the linear FIR filter is referred to in the art as “residue echo”. If the residue echo is lower than a predefined threshold, a Non-Linear Processor (NLP) can be used to replace the residue echo with comfort noise. However, reducing the residue echo to below this threshold is difficult using an online adaptive procedure, because the nonlinearity is buried in the training signal.
Clearly, a well-matched four-wire circuit gives little echo and less distortion, making the echo-canceling task easier. However, the selection of a best set of matching impedance settings for a specific LS trunk Line is currently very objective, mainly based on experience using trial and error. Such manual measurement consumes enormous human effort and time.